Generally, the more the audio signal to be reproduced has spatial information, the richer a sound field feeling or atmospheric feeling becomes when reproducing an audio signal, and the spatial information appears in the difference between the left and right signals (referred to as a left-and-right difference signal, from now on).
On the other hand, techniques have been spread recently which save the capacity of a storage device for storing audio signals or save the amount of communications of transmission and reception by carrying out compression encoding such as AAC (Advanced Audio Codec) or MP3 (MPEG Audio Layer 3) rather than by using audio CDs.
The compression-encoded audio signal has deteriorated characteristics like a tooth missing such as a lack of a high-frequency component and missing part of a middle- and high-frequency spectrum of the left-and-right difference signal. Playing back such an audio signal with its characteristics being deteriorated has a tendency to cause a muffled sound because of the lack of the high-frequency component, and a tendency to degenerate a sound field feeling and atmospheric feeling because of the deterioration in the characteristics of the left-and-right difference signal.
Accordingly, a signal processing device capable of improving the quality of sound of the compression-encoded audio signal is disclosed (see Patent Document 1). According to the Patent Document 1, it extracts a high-frequency component and low-frequency component of a peak value of an input audio signal and adds them, thereby being able to recover the high-frequency component missed because of the signal compression encoding and to lessen the muffled sound.